diff --git a/lib/libc/amd64/string/Makefile.inc b/lib/libc/amd64/string/Makefile.inc
--- a/lib/libc/amd64/string/Makefile.inc
+++ b/lib/libc/amd64/string/Makefile.inc
@@ -6,6 +6,7 @@
 	memccpy.S \
 	memcpy.S \
 	memmove.S \
+	memrchr.S \
 	memset.S \
 	stpcpy.S \
 	stpncpy.S \
diff --git a/lib/libc/amd64/string/memrchr.S b/lib/libc/amd64/string/memrchr.S
new file mode 100644
--- /dev/null
+++ b/lib/libc/amd64/string/memrchr.S
@@ -0,0 +1,166 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2023 Robert Clausecker
+ */
+
+#include <machine/asm.h>
+
+#include "amd64_archlevel.h"
+
+#define	ALIGN_TEXT	.p2align 4, 0x90
+
+ARCHFUNCS(memrchr)
+	ARCHFUNC(memrchr, scalar)
+	ARCHFUNC(memrchr, baseline)
+ENDARCHFUNCS(memrchr)
+
+ARCHENTRY(memrchr, scalar)
+	xor	%eax, %eax		# prospective return value
+	sub	$4, %rdx		# 4 bytes left to process?
+	jb	1f
+
+	ALIGN_TEXT
+0:	xor	%r8, %r8
+	lea	2(%rdi), %r10
+	cmp	%sil, 2(%rdi)
+	cmovne	%r8, %r10		# point to null if no match
+
+	cmp	%sil, (%rdi)
+	cmove	%rdi, %r8		# point to first char if match
+
+	lea	1(%rdi), %r9
+	cmp	%sil, 1(%rdi)
+	cmovne	%r8, %r9		# point to first result if no match in second
+
+	lea	3(%rdi), %r11
+	cmp	%sil, 3(%rdi)
+	cmovne	%r10, %r11
+
+	test	%r11, %r11
+	cmovz	%r9, %r11		# take first pair match if none in second
+
+	test	%r11, %r11
+	cmovnz	%r11, %rax		# take match in current set if any
+
+	add	$4, %rdi
+	sub	$4, %rdx
+	jae	0b
+
+1:	cmp	$-3, %edx		# a least one character left to process?
+	jb	2f
+
+	cmp	%sil, (%rdi)
+	cmove	%rdi, %rax
+
+	lea	1(%rdi), %rcx
+	cmp	$-2, %edx		# at least two characters left to process?
+	jb	2f
+
+	cmp	%sil, 1(%rdi)
+	cmove	%rcx, %rax
+
+	lea	2(%rdi), %rcx
+	cmp	$-1, %edx		# at least three character left to process?
+	jb	2f
+
+	cmp	%sil, 2(%rdi)
+	cmove	%rcx, %rax
+
+2:	ret
+ARCHEND(memrchr, scalar)
+
+ARCHENTRY(memrchr, baseline)
+	movd		%esi, %xmm4
+	test		%rdx, %rdx		# empty buffer?
+	jz		.L0			# if yes, return immediately
+
+	punpcklbw	%xmm4, %xmm4		# c -> cc
+	mov		%edi, %ecx
+	punpcklwd	%xmm4, %xmm4		# cc -> cccc
+	and		$~0xf, %rdi		# align source pointer
+	pshufd		$0, %xmm4, %xmm4	# cccc -> cccccccccccccccc
+	and		$0xf, %ecx
+	movdqa		%xmm4, %xmm0
+	mov		$-1, %r8d
+	pcmpeqb		(%rdi), %xmm0		# compare aligned head
+	shl		%cl, %r8d		# mask of bytes in the head of the buffer
+	pmovmskb	%xmm0, %eax
+
+	sub		$16, %rcx
+	and		%r8d, %eax		# match mask
+	add		%rcx, %rdx		# advance past head
+	cmc
+	jbe		.Lrunt			# did the string end in the buffer?
+
+	mov		%rdi, %rsi		# pointer to matching chunk
+	add		$16, %rdi
+	sub		$16, %rdx		# enough left for another round?
+	jbe		1f
+
+	/* main loop unrolled twice */
+	ALIGN_TEXT
+0:	movdqa		%xmm4, %xmm0
+	pcmpeqb		(%rdi), %xmm0
+	pmovmskb	%xmm0, %r8d
+
+	cmp		$16, %rdx		# enough left for second chunk?
+	jbe		2f
+
+	movdqa		%xmm4, %xmm0
+	pcmpeqb		16(%rdi), %xmm0
+	pmovmskb	%xmm0, %ecx
+
+	lea		16(%rdi), %r9
+	test		%ecx, %ecx		# match found in second chunk?
+	cmovz		%r8d, %ecx		# if not, use match data from first chunk
+	cmovz		%rdi, %r9
+
+	test		%ecx, %ecx		# any match found?
+	cmovnz		%ecx, %eax		# if yes, overwrite previously found match
+	cmovnz		%r9, %rsi
+
+	add		$32, %rdi		# advance to next iteration
+	sub		$32, %rdx		# advance to next chunks
+	ja		0b
+
+	/* process remaining 1--16 bytes */
+1:	pcmpeqb		(%rdi), %xmm4
+	mov		$0xffff, %r8d
+	xor		%ecx, %ecx
+	sub		%edx, %ecx		# number of bytes to be masked out
+	pmovmskb	%xmm4, %r9d
+	shr		%cl, %r8d		# mask of bytes to be kept in the buffer
+	and		%r9d, %r8d
+	cmovnz		%r8d, %eax
+	cmovnz		%rdi, %rsi
+	bsr		%eax, %eax
+	lea		(%rsi, %rax, 1), %rsi	# pointer to match (or junk)
+	cmovnz		%rsi, %rax		# if any match was found, return it
+	ret
+
+	/* end of chunk reached within first half iteration */
+2:	test		%r8d, %r8d		# match in previous chunk?
+	cmovnz		%r8d, %eax		# if yes, overwrite previous chunks
+	cmovnz		%rdi, %rsi
+	add		$16, %rdi		# point to tail 
+	sub		$16, %edx
+	jmp		1b			# handle tail the same otherwise
+
+	/* runt: string ends within head, edx has negated amount of invalid head bytes */
+.Lrunt:	mov		$0xffff, %r8d
+	xor		%ecx, %ecx
+	sub		%edx, %ecx
+	shr		%cl, %r8d
+	and		%r8d, %eax
+	bsr		%eax, %eax
+	lea		(%rdi, %rax, 1), %rdi
+	cmovnz		%rdi, %rax
+	ret
+
+	/* empty buffer: return a null pointer */
+.L0:	xor		%eax, %eax
+	ret
+ARCHEND(memrchr, baseline)
+
+	.section	.note.GNU-stack, "", %progbits
diff --git a/lib/libc/tests/string/Makefile b/lib/libc/tests/string/Makefile
--- a/lib/libc/tests/string/Makefile
+++ b/lib/libc/tests/string/Makefile
@@ -11,6 +11,7 @@
 ATF_TESTS_C+=		flsll_test
 ATF_TESTS_C+=		memccpy_test
 ATF_TESTS_C+=		memcmp_test
+ATF_TESTS_C+=		memrchr_test
 ATF_TESTS_C+=		memset_s_test
 ATF_TESTS_C+=		strncmp_test
 ATF_TESTS_C+=		stpncpy_test
diff --git a/lib/libc/tests/string/memrchr_test.c b/lib/libc/tests/string/memrchr_test.c
new file mode 100644
--- /dev/null
+++ b/lib/libc/tests/string/memrchr_test.c
@@ -0,0 +1,116 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2023 Robert Clausecker
+ */
+
+#include <sys/cdefs.h>
+
+#include <dlfcn.h>
+#include <limits.h>
+#include <string.h>
+
+#include <atf-c.h>
+
+static void *(*memrchr_fn)(const void *, int, size_t);
+
+ATF_TC_WITHOUT_HEAD(null);
+ATF_TC_BODY(null, tc)
+{
+	ATF_CHECK_EQ(memrchr_fn(NULL, 42, 0), NULL);
+}
+
+ATF_TC_WITHOUT_HEAD(not_found);
+ATF_TC_BODY(not_found, tc)
+{
+	size_t i, j;
+	char buf[1+15+64+1]; /* offset [0..15] + 64 buffer bytes + sentinels */
+
+	buf[0] = 'X';
+	memset(buf + 1, '-', sizeof(buf) - 1);
+
+	for (i = 0; i < 16; i++)
+		for (j = 0; j < 64; j++) {
+			buf[i + j + 1] = 'X';
+			ATF_CHECK_EQ(memrchr_fn(buf + i + 1, 'X', j), NULL);
+			buf[i + j + 1] = '-';
+		}
+}
+
+static void
+do_found_test(char buf[], size_t len, size_t first, size_t second)
+{
+	/* invariant: first <= second */
+
+	buf[first] = 'X';
+	buf[second] = 'X';
+	ATF_CHECK_EQ(memrchr_fn(buf, 'X', len), buf + second);
+	buf[first] = '-';
+	buf[second] = '-';
+}
+
+ATF_TC_WITHOUT_HEAD(found);
+ATF_TC_BODY(found, tc)
+{
+	size_t i, j, k, l;
+	char buf[1+15+64+1];
+
+	buf[0] = 'X';
+	memset(buf + 1, '-', sizeof(buf) - 1);
+
+	for (i = 0; i < 16; i++)
+		for (j = 0; j < 64; j++)
+			for (k = 0; k < j; k++)
+				for (l = 0; l <= k; l++) {
+					buf[i + j + 1] = 'X';
+					do_found_test(buf + i + 1, j, l, k);
+					buf[i + j + 1] = '-';
+				}
+}
+
+/* check that the right character is found */
+static void
+do_values_test(unsigned char buf[], size_t len, size_t i, int c)
+{
+	/* sentinels */
+	buf[-1] = c;
+	buf[len] = c;
+	memset(buf, c + 1, len);
+
+	if (i < len) {
+		buf[i] = c;
+		ATF_CHECK_EQ(memrchr_fn(buf, c, len), buf + i);
+	} else
+		ATF_CHECK_EQ(memrchr_fn(buf, c, len), NULL);
+}
+
+ATF_TC_WITHOUT_HEAD(values);
+ATF_TC_BODY(values, tc)
+{
+	size_t i, j, k;
+	int c;
+	unsigned char buf[1+15+64+1];
+
+	for (i = 0; i < 16; i++)
+		for (j = 0; j < 64; j++)
+			for (k = 0; k <= j; k++)
+				for (c = 0; c <= UCHAR_MAX; c++)
+					do_values_test(buf + i + 1, j, k, c);
+}
+
+ATF_TP_ADD_TCS(tp)
+{
+	void *dl_handle;
+
+	dl_handle = dlopen(NULL, RTLD_LAZY);
+	memrchr_fn = dlsym(dl_handle, "test_memrchr");
+	if (memrchr_fn == NULL)
+		memrchr_fn = memrchr;
+
+	ATF_TP_ADD_TC(tp, null);
+	ATF_TP_ADD_TC(tp, not_found);
+	ATF_TP_ADD_TC(tp, found);
+	ATF_TP_ADD_TC(tp, values);
+
+	return (atf_no_error());
+}
diff --git a/share/man/man7/simd.7 b/share/man/man7/simd.7
--- a/share/man/man7/simd.7
+++ b/share/man/man7/simd.7
@@ -24,7 +24,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE
 .
-.Dd December 4, 2023
+.Dd December 6, 2023
 .Dt SIMD 7
 .Os
 .Sh NAME
@@ -63,6 +63,7 @@
 .It    memccpy           Ta            Ta        Ta    S1
 .It    memcpy            Ta    S       Ta    S   Ta    S1     Ta    S    Ta    SV
 .It    memmove           Ta    S       Ta    S   Ta    S1     Ta    S    Ta    SV
+.It    memrchr           Ta            Ta        Ta    S1
 .It    memset            Ta    S       Ta    S   Ta    S      Ta    S
 .It    rindex            Ta    S       Ta        Ta    S1     Ta    S
 .It    stpcpy            Ta    S       Ta        Ta    S1